1. Field of the Invention
The present invention relates to a process of producing an oxide superconductor of the Y--Ba--Cu--O system (hereinafter simply referred to as "Y-based [oxide] superconductor") with well-controlled crystal orientation. The Y-based superconductor according to the present invention is applicable as superconducting bearings, superconducting electromagnetic transporters, superconducting permanent magnets, magnetic shields, etc.
2. Description of the Related Art
It is known that oxide superconductors, particularly Y-based superconductors, have an extremely improved critical current density when crystallized or solidified from a partially molten or semimolten state, than when produced by sintering (Jin et al., Appl. Phys. Lett. 52 (1988), p.2074).
New melting processes such as the "melt-powder-melt growth (MPMG) process" provide a fine dispersion of a normal conductor phase in a superconductor crystal to ensure a high critical current density even in a strong magnetic field (M. Murakami, Mod. Phys. Lett. B4 (1990), p.163 and U.S. Ser. No. 07/606,207).
The MPMG process includes heating and melting a powder of a Y-based superconductor composition at about 1300.degree. to 1450.degree. C., quenching the melt to form a solidified body, and pulverizing the solidified body to form a fine powder. A compact body formed from the fine powder is brought into a semimolten state and the semimelt is then slowly cooled from about 1000.degree. C. The thus-produced superconductor material exhibits a strong repulsive force against a magnet and is expected to be useful as a superconductor bearing, etc.
As oxide superconductors, particularly those having a perovskite structure, are anisotropic in property, the crystal orientation is desirably controlled for practical applications. For example, it is known that electric current preferentially flows along the a-or b-axis (hereinafter collectively referred to as "a/b-axis") of the crystal structure. Jin et al. achieved an improvement in the critical current density by utilizing a temperature gradient during the crystallization or solidification from semimelt and thereby forming a crystal structure oriented in the preferential direction.
The conventional processes using the temperature gradient, however, have not yet succeeded in perfect control of the crystal orientation. For example, FIG. 1 shows that the crystal growth proceeds with the ab-axis directed along the temperature gradient, but the c-axis is not fixed. This is fully acceptable for application to wires or tapes, which only require a current flow in the longitudinal direction, i.e., along the a/b-axis providing the preferential current flow path.
On the other hand, in applications requiring a two-dimensional current flow such as bearings, the direction of the c-axis must be also controlled in order to improve service performance. Moreover, members having a greater thickness than wires or tapes are subjected to the influence of temperature distribution over the thickness and thereby mostly include regions which are differently oriented.